Various topologies are known for DC-DC converters (also designated as DC choppers) for high output currents. Use is often made of full-bridge circuits having a split winding on the output side or a current doubler circuit.
With the use of intermediate circuit voltages of up to 850 V, IGBTs (insulated-gate bipolar transistors) having a dielectric strength of 1200 V are preferably used, which leads to a significant upward limitation of the switching frequency, as a result of which relatively large inductive components become necessary.
DE 202 21 373 U1 discloses a circuit in which two asymmetrical half-bridges operate on a split primary winding of a transformer. This enables the use of MOSFETs as switching elements, as a result of which the switching frequency can be considerably increased. On the output side, said circuit operates with a current doubler circuit. In order to control reverse current effects of the diodes, additional saturable reactors are necessary.
In the case of the circuit disclosed in DE 202 21 373 U1, polarity reversal of the output terminals, for example upon connection of a battery, inevitably leads to high currents and thus to the destruction of the rectifier diodes. A high current loading of the windings of the transformer furthermore results on account of the current doubler circuit.
The invention is based on the object of providing a DC-DC converter which enables the use of MOSFETs as switching elements and at the same time ensures a reliable polarity reversal protection and/or has a low current loading of the windings of the transformer.
The invention achieves this object by means of a DC-DC converter. The DC-DC converter comprises a first, in particular asymmetrical, half-bridge circuit and a second, in particular asymmetrical, half-bridge circuit.
Furthermore, at least one, in particular galvanically isolating, transformer is provided, comprising at least one primary winding and at least one secondary winding. The first and second half-bridge circuits are designed to generate an AC voltage at the at least one primary winding.
A rectifier circuit of the DC-DC converter comprises an output terminal, wherein the output terminal comprises a first output terminal pole or a first output terminal and a second output terminal pole or a second output terminal, and at least one rectifier element, wherein the rectifier circuit is designed to rectify a voltage present at the at least one secondary winding and to output it in a rectified fashion at the output terminal.
The rectifier circuit comprises a polarity reversal protection transistor, the collector-emitter path of which or the drain-source path of which is looped in between a terminal of the at least one rectifier element and the first or the second output terminal pole of the output terminal.
The polarity reversal protection transistor ensures a reliable polarity reversal protection, such that, for example, polarity reversal of the output terminal poles upon connection of a battery does not lead to destruction of the rectifier elements.
The DC-DC converter can comprise exactly one transformer, wherein the transformer comprises exactly two primary windings and exactly two secondary windings.
The first half-bridge circuit can be designed to generate an AC voltage at the first primary winding, and the second half-bridge circuit can be designed to generate an AC voltage at the second primary winding.
The rectifier circuit can comprise exactly two rectifier elements in the form of a first diode and a second diode, wherein a first terminal of the first secondary winding is electrically connected to the anode of the first diode, and the drain-source path of the MOSFET polarity reversal protection transistor and an inductor (in any order) are looped in between the cathode of the first diode and the first output terminal pole of the output terminal.
A first terminal of the second secondary winding can be electrically connected to the anode of the second diode, the cathode of the first diode and the cathode of the second diode can be electrically connected, and the second terminal of the first secondary winding, the second terminal of the second secondary winding and the second output terminal pole of the output terminal can be electrically connected.
The DC-DC converter can comprise exactly two, for example magnetically coupled, transformers, wherein the two transformers each comprise exactly one primary winding and exactly one secondary winding. The first half-bridge circuit can be designed to generate an AC voltage at the primary winding of the first transformer, and the second half-bridge circuit can be designed to generate an AC voltage at the primary winding of the second transformer. The rectifier circuit can comprise a first pair of rectifier elements in the form of a first diode and a second diode and comprise a second pair of rectifier elements in the form of a third diode and a fourth diode. A first terminal of the secondary winding of the first transformer can be electrically connected to the anode of the first diode, a first terminal of the secondary winding of the second transformer can be electrically connected to the anode of the third diode, the cathodes of the first to fourth diodes can be electrically connected, and the drain-source path of the MOSFET polarity reversal protection transistor and an inductor (in any order) can be looped in between the cathodes of the first to fourth diodes and the first output terminal pole of the output terminal.
A second terminal of the secondary winding of the first transformer, a second terminal of the secondary winding of the second transformer, the anodes of the second diode and of the fourth diode and the second output terminal pole of the output terminal can be electrically connected.
The DC-DC converter can comprise exactly one transformer, wherein the transformer comprises exactly two primary windings and exactly one secondary windings. The first half-bridge circuit can be designed to generate an AC voltage at the first primary winding, and the second half-bridge circuit can be designed to generate an AC voltage at the second primary winding. The rectifier circuit can comprise two rectifier elements in the form of a first diode and a second diode, wherein a first terminal of the secondary winding is electrically connected to the cathode of the first diode, the anode of the first diode is electrically connected to the anode of the second diode, the cathode of the second diode is electrically connected to a second terminal of the secondary winding, and the drain-source path of the polarity reversal protection transistor is looped in between the anodes of the first and second diodes and the second output terminal pole of the output terminal.
A first inductor can be looped in between the first terminal of the secondary winding and the first output terminal pole of the output terminal, and a second inductor can be looped in between the second terminal of the second secondary winding and the first output terminal pole of the output terminal.
MOSFETs (metal oxide semiconductor field effect transistors) can be used as respective switching means in the first and second half-bridge circuits.
All the rectifier elements and the polarity reversal protection transistor can be integrated into a power module.